Sessions

Internal engineering reference for Sillo’s session subsystem.

Source: core/sillo/session/ (7 files, ~844 lines)

---

1. Overview and Architecture

The session subsystem provides per-request key-value storage persisted across HTTP round-trips. It follows the classic middleware-interceptor pattern: one middleware reads a cookie on the way in and writes a cookie on the way out; the handler sees a plain dict-like object (request.session) and never touches cookies, backends, or signing directly.

Architecture Diagram

classDiagram
    class SessionMiddleware {
        +SessionConfig session_config
        +BaseSessionInterface session_interface
        +process_request(request, response, call_next)
        +process_request(request, response)
    }
    class SessionConfig {
        -dict _config
        +session_cookie_name: str
        +session_expiration_time: int
        +session_permanent: bool
        +session_refresh_each_request: bool
        +session_cookie_secure: bool
        +session_cookie_httponly: bool
        +session_cookie_samesite: str
        +session_cookie_path: str
        +session_cookie_domain: str|None
        +session_file_storage_path: str|None
        +manager: Any|None
    }
    class BaseSessionInterface {
        +config: SessionConfig
        +generate_session_key() str
        +create_session(session_key) Session
        +load(session)*
        +save(session)*
        +get_cookie_name() str
        +get_cookie_domain()
        +get_cookie_path() str
        +get_cookie_httponly() bool
        +get_cookie_secure() bool
        +get_cookie_samesite() str
    }
    class Session {
        -dict _session_cache
        +modified: bool
        +accessed: bool
        +deleted: bool
        +get_session_key() str
        +should_set_cookie: bool
        +load()
        +save() str
    }
    class SignedSessionManager {
        +URLSafeTimedSerializer serializer
        +sign_session_data(data) str
        +verify_session_data(token) dict
        +load(session)
        +save(session) str
    }
    class FileSessionInterface {
        +storage_path: str
        +load(session)
        +save(session) str
    }

    SessionMiddleware --> SessionConfig
    SessionMiddleware --> BaseSessionInterface
    Session --> BaseSessionInterface
    BaseSessionInterface <|-- SignedSessionManager
    BaseSessionInterface <|-- FileSessionInterface
    SignedSessionManager --> Session : creates
    FileSessionInterface --> Session : creates

Request/Response Flow

sequenceDiagram
    participant Client
    participant Middleware as SessionMiddleware
    participant Session as Session Object
    participant Backend as SessionInterface
    participant Handler

    Client->>Middleware: HTTP Request (Cookie: session_id=abc123)
    Note over Middleware: process_request
    Middleware->>Middleware: Read cookie from request
    Middleware->>Backend: create_session("abc123")
    Backend-->>Middleware: Session(interface, "abc123")
    Middleware->>Session: await session.load()
    Session->>Backend: await interface.load(session)
    Backend-->>Session: Populate _session_cache
    Middleware->>Handler: call_next() (session in scope)

    Handler->>Session: request.session["user_id"] = 42
    Note over Session: modified=True, accessed=True

    Handler-->>Middleware: Response
    Note over Middleware: process_response
    Middleware->>Session: Check should_set_cookie
    Middleware->>Session: await session.save()
    Session->>Backend: await interface.save(session)
    Backend-->>Session: Returns cookie value
    Middleware->>Client: Set-Cookie: session_id=<value>

File Inventory

File	Path	Lines	Purpose
__init__.py	core/sillo/session/__init__.py	4	Public API re-exports
config.py	core/sillo/session/config.py	202	SessionConfig, reject_unknown_settings
base.py	core/sillo/session/base.py	63	BaseSessionInterface ABC
session_objects.py	core/sillo/session/session_objects.py	316	Session dict-like object
signed_cookies.py	core/sillo/session/signed_cookies.py	57	SignedSessionManager
file.py	core/sillo/session/file.py	74	FileSessionManager
middleware.py	core/sillo/session/middleware.py	129	SessionMiddleware

---

2. SessionConfig

File: core/sillo/session/config.py, line 65

SessionConfig is a typed container for every session-related setting. It exists so that session settings are gathered in one place and validated eagerly rather than silently ignored.

Recognised Settings

The module-level SETTINGS tuple (line 7) defines the canonical setting names:

session_cookie_name          session_cookie_httponly
session_expiration_time      session_cookie_samesite
session_permanent            session_cookie_path
session_refresh_each_request session_cookie_domain
session_cookie_secure        session_file_storage_path
manager

Rejecting Unknown Settings

# core/sillo/session/config.py, line 22
def reject_unknown_settings(names: Iterable[str], *, called: str) -> None:

Called from both SessionConfig.__init__ and SessionMiddleware.__init__ with any **kwargs that were not consumed by explicit parameters.

How it works:

1. Compares each supplied name against the SETTINGS tuple.
2. For any name not found, uses difflib.get_close_matches(name, SETTINGS, n=3, cutoff=0.5) to suggest corrections.
3. Raises TypeError with a message listing the unknown name(s) and suggestions.

Motivation: Prevents silent misconfiguration. A user who writes cookie_secure=False instead of session_cookie_secure=False gets an immediate error with a helpful “did you mean” suggestion rather than a silently insecure cookie.

# Example error message:
# TypeError: Unknown session setting 'cookie_secure'.
# Did you mean: session_cookie_secure?

Constructor

# core/sillo/session/config.py, line 71
def __init__(
    self,
    session_cookie_name: str = "session_id",
    session_expiration_time: int = 86400,
    session_permanent: bool = True,
    session_refresh_each_request: bool = True,
    session_cookie_secure: bool = True,
    session_cookie_httponly: bool = True,
    session_cookie_samesite: str = "lax",
    session_cookie_path: str = "/",
    session_cookie_domain: str | None = None,
    session_file_storage_path: str | None = None,
    manager: Any | None = None,
    **kwargs: Any,
)

All parameters are stored in self._config: dict[str, Any]. The **kwargs are passed to reject_unknown_settings for validation before storage.

Attribute Access

__getattr__ (line 129) provides dot-notation access into _config. Names starting with _ are rejected with AttributeError to avoid confusion with private attributes. Missing names raise AttributeError listing all valid settings.

Properties

Each setting has a dedicated property (lines 149-202) that reads from _config with a fallback default:

Property	Type	Default
session_cookie_name	str	"session_id"
session_expiration_time	int	86400 (24 hours)
session_permanent	bool	True
session_refresh_each_request	bool	True
session_cookie_secure	bool	True
session_cookie_httponly	bool	True
session_cookie_samesite	Literal["lax","strict","none"]	"lax"
session_cookie_path	str	"/"
session_cookie_domain	str | None	None
session_file_storage_path	str | None	None
manager	Any | None	None

Serialisation

to_dict() (line 146) returns a shallow copy of the internal _config dict, useful for logging or debugging.

---

3. Session Object

File: core/sillo/session/session_objects.py, line 6

Session is the per-request session object stored at request.session. It behaves like a dictionary but tracks three boolean flags that determine how the middleware handles it in the response phase.

Internal State

Attribute	Type	Purpose
interface	BaseSessionInterface	The backend that persists this session
session_key	str | None	Cookie value (or None for new visitors)
_session_cache	dict[str, Any]	In-memory data store
modified	bool	Set True by writes and deletes
accessed	bool	Set True by reads
deleted	bool	Set True by deletes and clear()
_expiration_time	datetime | None	Explicit override for expiration

Dict-Like Protocol

Every read method sets accessed = True. Every write method sets modified = True (and accessed = True). Every delete sets deleted = True.

Method	Line	Flags Set	Raises on Missing
__getitem__(key)	50	accessed	KeyError
__setitem__(key, value)	66	modified, accessed
__delitem__(key)	79	modified, deleted	KeyError
__contains__(key)	93	accessed
__len__()	105	accessed
__iter__()	309	accessed
get(key, default)	110	accessed
set(key, value)	123	modified, accessed
delete(key)	134	modified, deleted	Never raises
clear()	145	accessed, modified, deleted
keys()	157	accessed
values()	162	accessed
items()	167	accessed
update(other)	180	modified
is_empty()	172	(none)

Design note: is_empty() intentionally does not set accessed so that middleware can check emptiness without triggering a cookie refresh.

Session Key Management

# core/sillo/session/session_objects.py, line 189
def get_session_key(self) -> str:
    if self.session_key:
        return self.session_key
    self.session_key = self.interface.generate_session_key()
    return self.session_key

For new visitors (no cookie), the key is lazily generated on first access via secrets.token_hex(32). A 64-character hex string with 256 bits of entropy.

Expiration

Three methods control when a session expires:

• set_expiration_time(expiration: datetime) (line 200): Explicit override.
• get_expiration_time(): Resolution order:
  1. Explicit override from set_expiration_time
  2. If session_permanent is True (the default): now(utc) + session_expiration_time
  3. If session_permanent is False: None — the cookie goes out with no Expires at all, so the browser drops it when it closes
  4. Fallback: session_expiration_time’s default of 86400 if config is unreachable
• has_expired(): now(utc) > get_expiration_time(), and False for a session with no expiry

This used to be inverted — a permanent session got datetime.max, so the default configuration issued a cookie that never expired, and session_expiration_time applied only to the non-permanent case that did not need it.

The cookie’s expiry is not what protects you

Expires is a request to the browser. Anyone replaying a captured cookie simply does not honour it. For the signed-cookie backend the real bound is SignedSessionManager.max_age, which is checked against a timestamp inside the signature and defaults to session_expiration_time.

should_set_cookie

# core/sillo/session/session_objects.py, line 260
@property
def should_set_cookie(self) -> bool:
    return self.modified or (
        self.config.session_permanent and self.config.session_refresh_each_request
    )

Returns True when the cookie needs to be written. Two triggers:

1. The session data was modified during the request.
2. The session is permanent and session_refresh_each_request is enabled (rolling expiry refresh).

Backend Delegation

# core/sillo/session/session_objects.py, line 280
async def load(self) -> None:
    await self.interface.load(self)

# core/sillo/session/session_objects.py, line 289
async def save(self) -> str:
    self.modified = False
    self.deleted = False
    self.accessed = False
    return await self.interface.save(self)

save() resets all flags before delegating so the backend can inspect them if needed, but the middleware’s decision has already been made.

---

4. BaseSessionInterface

File: core/sillo/session/base.py, line 6

The abstract base class that all session backends must implement.

Constructor

def __init__(self, config: SessionConfig | None = None) -> None:
    self.config = config

Concrete Methods

Method	Line	Returns	Notes
generate_session_key()	13	secrets.token_hex(32)	64 hex chars, 256 bits
create_session(session_key)	17	Session(self, session_key)	Factory method
get_cookie_name()	29	config.session_cookie_name or "session_id"
get_cookie_domain()	35	config.session_cookie_domain or None
get_cookie_path()	41	config.session_cookie_path or "/"
get_cookie_httponly()	47	config.session_cookie_httponly or True
get_cookie_secure()	53	config.session_cookie_secure or False
get_cookie_samesite()	59	config.session_cookie_samesite or "lax"

Note: The cookie getter methods are utility methods on the base class. The middleware reads cookie attributes directly from SessionConfig rather than calling these methods.

Abstract Methods

async def load(self, session: Session) -> None:
    raise NotImplementedError

async def save(self, session: Session) -> str:
    raise NotImplementedError

• load: Populate session._session_cache from the backend. For server-side stores, this means reading from disk/database/Redis using session.session_key. For cookie-based stores, this means verifying and deserialising the cookie value.
• save: Persist session._session_cache to the backend. Return the string to be stored in the cookie. For server-side stores, this is the session key. For cookie-based stores, this is the signed serialised data.

---

5. SessionMiddleware

File: core/sillo/session/middleware.py, line 13

Constructor Resolution

# core/sillo/session/middleware.py, line 16
def __init__(
    self,
    config: SessionConfig | None = None,
    manager: BaseSessionInterface | None = None,
    secret_key: str | None = None,
    **settings: Any,
)

The constructor resolves the session backend through a priority chain:

flowchart TD
    A["Explicit manager= argument?"] -->|Yes| B["Use manager"]
    A -->|No| C["config.manager set?"]
    C -->|Yes| D["Use config.manager"]
    C -->|No| E["Default: SignedSessionManager(secret_key)"]
    B --> F{"Is it an instance?"}
    D --> F
    E --> F
    F -->|Yes| G["Store as session_interface"]
    F -->|No class| H["TypeError: Pass an instance, not a class"]

Key validation steps:

1. Unknown settings (line 44): Calls reject_unknown_settings(settings, called="SessionMiddleware()").
2. Mutual exclusion (line 50): Raises TypeError if both config= and keyword settings are provided.
3. Config creation (line 58): self.session_config = config or SessionConfig(**settings).
4. Manager resolution (line 61-74): Uses the priority chain above. Raises TypeError if the resolved value is a class rather than an instance, with a helpful message showing the correct instantiation pattern.

process_request

# core/sillo/session/middleware.py, line 82
async def process_request(self, request, response, call_next):

1. Reads the cookie name from self.session_config.session_cookie_name.
2. Gets session_key from request.cookies.get(cookie_name).
3. Creates a Session via self.session_interface.create_session(session_key).
4. Calls await session.load() to populate from the backend.
5. Stores in request.scope["session"].
6. Calls await call_next() to proceed to the handler.

process_response

# core/sillo/session/middleware.py, line 99
async def process_response(self, request, response):

Three cases:

1. No session in scope (line 103): Returns early.
2. Empty session that was accessed and modified (lines 107-115): Calls await session.save() to let the backend purge its record, then response.delete_cookie(cookie_name). This handles logout. The server-side store is cleaned up and the cookie is removed.
3. should_set_cookie is true (lines 117-129): Calls await session.save() to get the cookie value, then sets the cookie with all configured attributes:
   • key: cookie name
   • value: returned from save()
   • domain, path, httponly, secure, samesite: from config
   • expires: from session.get_expiration_time()

---

6. SignedSessionManager

File: core/sillo/session/signed_cookies.py, line 8

A cookie-based session store with no server-side state. The entire session payload is signed and serialised into the cookie value itself.

Cryptographic Foundation

# core/sillo/session/signed_cookies.py, line 11
def __init__(self, config=None, secret_key: str | None = None):
    if not secret_key:
        raise RuntimeError("secret_key is required for SignedSessionManager")
    self.serializer = URLSafeTimedSerializer(
        secret_key=secret_key,
        salt="nexio.session.signed_cookie",
    )

Uses an internal URLSafeTimedSerializer which:

• Serialises the payload to JSON, then base64url-encodes it.
• Appends a timestamp.
• Signs the whole thing with HMAC-SHA256 using the secret key and salt.
• The salt prevents signature reuse across different signing contexts.

Signing and Verification

def sign_session_data(self, session_data: dict[str, Any]) -> str:
    return self.serializer.dumps(session_data)

def verify_session_data(self, token: str | None) -> dict[str, Any]:
    if not token:
        return {}
    try:
        return self.serializer.loads(token)
    except BadSignature:
        return {}

verify_session_data silently returns an empty dict on bad signatures, which creates a fresh (anonymous) session rather than crashing.

Load and Save

async def load(self, session):
    # session.session_key IS the signed token
    data = self.verify_session_data(session.session_key)
    if data:
        session._session_cache = data
    else:
        session._session_cache = {}

async def save(self, session) -> str:
    if session.deleted:
        session.session_key = ""
        return ""
    signed = self.sign_session_data(session._session_cache)
    session.session_key = signed
    return signed

Key insight: The cookie value is the entire session payload, not a reference to server-side storage. This means:

• No disk I/O, no database queries: the fastest possible session backend.
• Size limit: browsers limit cookies to ~4 KB; large session data will exceed this.
• No server-side revocation: you cannot invalidate a session without waiting for the cookie to expire (or using a separate revocation list).

When to Use

• Small session data (user ID, role, flash messages).
• Horizontally scaled deployments where shared server-side state is inconvenient.
• Applications where session speed is critical.

When NOT to Use

• Session data exceeding ~3 KB (after base64 encoding).
• Need for server-side session revocation.
• Sensitive data that should not reside in cookies (even signed).

---

7. FileSessionManager

File: core/sillo/session/file.py, line 8

A server-side session store using JSON files on disk.

Constructor

def __init__(self, config=None):
    super().__init__(config)
    self.storage_path = getattr(
        config, "session_file_storage_path", None
    ) or "__sessions"
    os.makedirs(self.storage_path, exist_ok=True)

File Operations

Method	Line	Operation
_get_file_path(key)	19	os.path.join(storage_path, f"{key}.json")
_load_session_data(key)	23	Read JSON from file; return None on missing/error
_save_session_data(key, data)	36	Write JSON to file
_delete_session_file(key)	43	os.remove() if exists

Load and Save

async def load(self, session):
    if session.session_key:
        data = self._load_session_data(session.session_key)
        if data is not None:
            session._session_cache = data
        else:
            session._session_cache = {}
    else:
        session._session_cache = {}

async def save(self, session) -> str:
    if session.deleted:
        self._delete_session_file(session.session_key)
        session.session_key = None
        return ""
    key = session.get_session_key()
    self._save_session_data(key, session._session_cache)
    session.session_key = key
    return key

Design difference from SignedSessionManager: The cookie contains only the session key (a 64-char hex string). The actual data lives in {storage_path}/{key}.json.

File Layout

__sessions/
  a1b2c3d4e5f6...json    # session data as JSON
  f9e8d7c6b5a4...json
  ...

Each file contains the raw _session_cache dict serialised via json.dumps.

Limitations

• No concurrency control: concurrent requests for the same session can race. A production deployment should use a proper database or Redis.
• No automatic cleanup: expired session files accumulate. A cron job or periodic cleanup is needed.
• Filesystem coupling: not suitable for multi-server deployments without shared storage.

---

8. Extension Points

Custom Backend

To create a custom session backend, subclass BaseSessionInterface and implement load and save:

from sillo.session.base import BaseSessionInterface
from sillo.session.session_objects import Session

class RedisSessionManager(BaseSessionInterface):
    def __init__(self, config=None, redis_client=None):
        super().__init__(config)
        self.redis = redis_client

    async def load(self, session: Session) -> None:
        if session.session_key:
            data = await self.redis.get(f"session:{session.session_key}")
            if data:
                session._session_cache = json.loads(data)
            else:
                session._session_cache = {}
        else:
            session._session_cache = {}

    async def save(self, session: Session) -> str:
        if session.deleted:
            if session.session_key:
                await self.redis.delete(f"session:{session.session_key}")
            session.session_key = ""
            return ""
        key = session.get_session_key()
        await self.redis.setex(
            f"session:{key}",
            self.config.session_expiration_time or 86400,
            json.dumps(session._session_cache),
        )
        session.session_key = key
        return key

Registering a Custom Backend

from sillo.session.middleware import SessionMiddleware
from sillo.session.config import SessionConfig

config = SessionConfig(session_cookie_name="app_session")
manager = RedisSessionManager(config=config, redis_client=redis)

app.use(SessionMiddleware(config=config, manager=manager))

Or via the config shortcut:

config = SessionConfig(manager=RedisSessionManager(redis_client=redis))
app.use(SessionMiddleware(config=config))

---

9. Testing Considerations

Unit Testing SessionConfig

def test_reject_unknown_settings():
    with pytest.raises(TypeError, match="cookie_secure"):
        SessionConfig(cookie_secure=False)

def test_valid_settings():
    config = SessionConfig(session_cookie_secure=False)
    assert config.session_cookie_secure is False

Unit Testing Session Object

def test_session_flags():
    interface = BaseSessionInterface()
    session = Session(interface)

    # Read sets accessed
    session._session_cache = {"key": "value"}
    _ = session["key"]
    assert session.accessed is True
    assert session.modified is False

    # Write sets modified
    session["key"] = "new"
    assert session.modified is True

    # Delete sets deleted
    del session["key"]
    assert session.deleted is True

Integration Testing with TestClient

from sillo.testclient import TestClient

def test_session_persists():
    with TestClient(app) as client:
        # First request sets session data
        resp = client.post("/login", json={"user": "admin"})
        assert resp.status_code == 200

        # Second request reads session data
        resp = client.get("/dashboard")
        assert resp.status_code == 200
        assert "admin" in resp.text

Testing SignedSessionManager

def test_signed_session_tamper():
    manager = SignedSessionManager(secret_key="test-secret")
    session = manager.create_session()
    session._session_cache = {"user": "admin"}

    # Save produces a signed token
    token = await session.save()
    assert token != ""

    # Tampered token produces empty session
    tampered = token[:-5] + "XXXXX"
    session2 = manager.create_session(tampered)
    await manager.load(session2)
    assert session2._session_cache == {}

Testing FileSessionManager

import tempfile

def test_file_session_lifecycle():
    with tempfile.TemporaryDirectory() as tmpdir:
        config = SessionConfig(session_file_storage_path=tmpdir)
        manager = FileSessionManager(config=config)

        session = manager.create_session()
        session["user"] = "test"
        key = await manager.save(session)

        # Reload from disk
        session2 = manager.create_session(key)
        await manager.load(session2)
        assert session2["user"] == "test"

        # Delete
        session2.deleted = True
        await manager.save(session2)
        session3 = manager.create_session(key)
        await manager.load(session3)
        assert session3._session_cache == {}

---

Appendix: Configuration Quick Reference

from sillo.session import SessionConfig, SessionMiddleware

# Minimal (uses SignedSessionManager by default)
app.use(SessionMiddleware(secret_key="your-secret-key"))

# With explicit config
config = SessionConfig(
    session_cookie_name="app_session",
    session_expiration_time=3600,       # 1 hour
    session_permanent=False,            # browser-session cookie, no Expires
    session_refresh_each_request=False, # don't refresh on each request
    session_cookie_secure=True,         # HTTPS only
    session_cookie_httponly=True,       # no JavaScript access
    session_cookie_samesite="strict",   # strict CSRF protection
    session_cookie_path="/",
    session_cookie_domain=".example.com",
)
app.use(SessionMiddleware(config=config, secret_key="your-secret-key"))

# With file-based sessions
config = SessionConfig(
    session_file_storage_path="/tmp/sessions",
    manager=FileSessionManager(),
)
app.use(SessionMiddleware(config=config))